<div class="header_menu col-md-9 col-sm-9 col-xs-12">
    <nav class="navbar navbar-default" role="navigation">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">MENU</a>
        </div>

        <!-- Collect the nav links, forms, and other content for toggling -->
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <?php wp_nav_menu(array(
                'theme_location'=>'main_nav',
                'depth'=>'2',
                'container'=>'',
                'menu_class'=>'menu',
                'walker'=>new twitter_bootstrap_nav_walker()));
            ?>
        </div><!-- /.navbar-collapse -->
    </nav>
</div>